Delayed hypersensitivity is a bodily reaction to an antigen that does not occur immediately, often involving T cells. It can be a chronic condition or a delayed response to allergens, but can also be beneficial, such as in tuberculin skin tests. Immediate hypersensitivity reactions are often due to allergies and can be dangerous. Anti-rejection drugs are often given to prevent organ failure due to delayed hypersensitivity.
Delayed hypersensitivity is difficult to understand without a working definition of hypersensitivity. In a medical sense, as opposed to an emotional meaning, hypersensitivity is a strong bodily reaction to some form of antigen. An antigen is a substance introduced into the body or perceived as foreign by the body. The body reacts to the antigen in perceptible ways, such as developing a rash, developing asthma, developing allergy symptoms, or showing signs of contagion from a disease, but sometimes this reaction does not occur immediately and is delayed hypersensitivity .
Hypersensitivity is often described in four classes/types. The fourth class is delayed hypersensitivity. Usually this delay is due to the way the body reacts. The extra time taken for the reaction tends to mean that T cells are involved in the response.
It’s easy to think of examples of delayed hypersensitivity. When people touch poison oak or ivy, it may take at least a day for the skin to react. Around this time, T cells have been working to create the rash that is the hallmark of exposure to poison oak or ivy antigens. Most people exposed to illnesses also don’t get them right away, but can develop them within days or weeks.
Immediate hypersensitivity reactions are often due to allergies. A person with a peanut allergy could develop severe allergies within seconds of consuming a single peanut. This rapid histamine response can be very dangerous and is usually not delayed in any way. Immediate treatment is needed to avoid anaphylactic shock.
A complicated aspect of delayed hypersensitivity can be a chronic condition or a constant reaction to antigens that takes days to fully express. However, when the expression begins to manifest itself, it seems to be an immediate response. An example of this is organ rejection.
When a person first receives a donor organ, rejection may not appear to be occurring, and open expression of rejection may take several days as the T cells marshal their forces. Once the rejection has settled in, it may still be a delayed reaction, even though clearly the organ is starting to fail. While the reaction is delayed hypersensitivity, organ failure occurs simultaneously, making it seem as though the reaction is in its current state instead of a process that started a few days ago. To prevent this, anti-rejection drugs are often given before the T-cell response can occur, leading to fewer cases of organ failure.
Delayed hypersensitivity isn’t always a bad thing. When people have tuberculin skin tests, they should have the results checked about a week later for evidence of exposure to tuberculosis. The delayed reaction is simply accepted as how the body responds to the test and to the antigen; it’s neither good nor bad, as long as people go back to the doctor to have the results interpreted. Also, as with organ transplants, it may be possible to intervene medically between exposure and reaction to certain substances, reducing the chances of the severity of the reaction.
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